Method of and apparatus for aiding hearing and the like

ABSTRACT

This disclosure deals with electronically aiding sensori-neural deafness with frequency-segmented, dynamic range-compressed speech signal processing, wherein noise vs. speech signal discrimination is employed with an optional semi-remote microphone input, and with an optional electronic frequency-shift processing of the signal to prevent or reduce oscillation due to acoustic airborne and/or vibrational feedback between the earphone(s) and the microphone(s).

United States Patent Kryter July s, 1975 METHOD OF AND APPARATUS FORAIDING HEARING AND THE LIKE Primary ExaminerRalph D. Blakeslee Attorney,Agent, or FirmRines and Rines; Shapiro [76] Inventor: Karl D. Kryter,13725 Robleda Rd.,

Los Altos, Calif. 94022 and Shapm 22 F'] d: 12, 1974 1 June 57 ABSTRACT21 A 1. N 478 462 1 pp 0 This disclosure deals with electronicallyaiding sensori-neural deafness with frequency-segmented, dy- [52] US.Cl. 179/107 FD a i ran eompressed peech signal processing,

Int. wherein noise vs speech signal discrimination is em- Field ofSearch 107 FD, 107 R, 107 ployed with an optional semi-remote microphonein- 179/1 D, 1 F5 put, and with an optional electronic frequency-shiftprocessing of the signal to prevent or reduce oscillal l ReferencesCited tion due to acoustic airborne and/or vibrational feed- UNITEDSTATES PATENTS back between the earphone(s) and the micro- 2,112,5693/1938 Lybarger 179/107 FD p 3,231,686 1/1966 Hueber 179/107 PD 1 l 23,243,525 3/1966 Eaton 179/107 FD 2 C Drawmg Flglres MICROPHONESEARPHONES gPTIONL LEFT RIGHT LEFT RIGHT LE RE VART/ TBLE AMPLIIQIIERS jAGC 1A F8 SECTIONI w BROAD BA -200-70001l2 2A sEcT|oN2 115%? N 287510-1500111 LGC A SECTION3 wa 35 1500-2500111 NLGC M 4A SECTION4 TT'G'EQ 48 2500-7000111 NLGC "ITFIFFHJUL 81975 3, 94 195 MICRO/PQONESEARPHONES 6PTI0NAL LEFT RIGHT LEFT RIGHT RE OTE LE RE VARIABLE GAINAMPLIFIERS AGC v 1A D E i sEcTIoNI BROAD BAND 1B -2OO-7OOOH2 W 2ASECTION 2 BAND-PASS 2B FILTER NLGC 75O-15OOH2 3A SECTION3 BAND-PASS 3BFILTER NLGC 1500-25OOHz SECTION 4 1: BAND-PASS Lg FILTER NLGC 4B%'\/W,

F NLGC SECTION 4 SECTION4 I l BAND-PASS I FILTER I PATH A I OUTPUT2500-7000Hz r I ATTACK- ,2 I RELEASE GATE TIME 2 CONTROL i R 1" I fifiREcTIFIER TIME 1 I CONTROL I B y FIg. Z.

METHOD OF AND APPARATUS FOR AIDING HEARING AND THE LIKE The presentinvention relates to methods of and apparatus fo r electronically aidinghearing or similar applications, being more particularly directed toimproving noise vs. speech signal discrimination.

The most prevalent type of deafness is so-called sensori-neural hearingloss, wherein the inner ear loses some ability to perceive the weakerintensity portions of the speech signal and also loses some ability tomake normal discriminations among some frequency components even thoughof sufficient intensity to be audible to the person with sensori-neutralhearing loss. Usually these losses in hearing ability are greater forthe higher sound frequencies, say, 2000 Hertz) than for the lower(below, say 2000 Hertz). The sensori-neural deafened ear, moreover,causes the perception of sounds that are very intense as excessivelyloud. Distortions not formed in the normal inner ear, which contains thesensorineural receptors, moreover, apparently occur in thesensori-neural deafened ear and result in less discrimi nation thannormal among the various speech sounds.

There are many electronic hearing aids which provide means forincreasing the intensity of the speech signal reaching the inner car sothat the weakened sounds are audible to the deafened ear. These hearingaids, however, while of help to persons suffering socalled conductivetype deafness, are not very helpful to sensori-neural deafnessbecause'of the aforementioned loss in discrimination ability, andbecause of the innerear distortions and excessive loudnesses that occurwhen sound amplification is applied to the strong as well as weakcomponents of the varying intensity speech signal in order that theweaker sounds be made audible to the sensori-neural ear. For example, aword such as show contains the consonant sh, which is much weaker thanthe vowel sound ow. A hearing aid that sufficiently amplifies all thesounds uniformly or linearly so that the weaker sh component, orphoneme, as it is called, is audible to the sensorineural ear, may alsomake the ow portion of the word extraordinarily loud and causedistortion in the inner ear, thereby tending to lessen understanding ofthe speech signal. It is also important to note that these weakerphonemes tend to have durations ranging from about 0.01 to less than 0.5second. It has been discovered, in accordance with the presentinvention, that effective use can be made of the relative difference inamplitude of segments of the speech signal and the relatively shortduration of the speech segments of phonemes, particularly the lessintense phonemes, to produce the improved results herein described.

In attempts to overcome the deficiency of linear-gain hearing aids,automatic non-linear or compression gain control systems have sometimesbeen used wherein the intensity of the speech signal is averaged for abrief period of time and this information is used automatically toadjust the gain of the amplifier. If the level is too low, the gain ofthe amplifier is increased by an amount proportional to the degree theaverage input voltage (over some specified period of time) falls below aspecified level. This process is called dynamic range compression; butit is difficult satisfactorily to achieve with speech signals becausethe signal level changes so quickly from one speech sound to another.Changing the gain without an adequate determination of the averageenvelope will cause distortion of the signal waveform and therebydegrade its understandability. In brief, an automatic gain controlsystem that more or less continuously (or too frequently) modifies thedegree of gain will tend to introduce distortion and as a result willnot always make the speech signal more understandable, as described byE. Trinder, An Attempt to Correct Speech Discrimination Loss in CochlearDeafness by Graded Instantaneous Compression, Sound, Vol. 5, pp. 62-67,(1972). Conversely, maintaining a given gain for too long a period oftime will also degrade the understandability of the speech signalbecause the gain setting will be inappropriate over significant segmentsof the speech phonemes wherein the level changes are very rapid.

Another shortcoming of automatic compression gain control systems isthat during periods of time when there is a pause in the input speechsignal, the gain control is progressively increased to a maximum amountand thereby tends to make objectionable to the hearing aid user, thenormally low level, or residual, noise present at the input of inherentin the electronics of the hearing aid. It is noted that in the presenthearing aid invention, as will be described later, an automaticnonlinear-linear gain control (to be labelled NLGC) device is utilizedthat has the ability to discriminate to a degree between speech signalsand background noise and adjust the system gain appropriately on thebasis of this information; i.e., prevent excessive amplification to theweak noise segments.

It might be noted that some reduction in the distortions that occur withautomatic compression gain can be reduced to some extent by theapplication of independent automatic compression gain controls todifferent portions of the speech spectrum; the amount, if any, for eachportion being adjusted to meet the degree and kind of hearing lossexperienced by a given ear with a sensori-neural hearing loss. Suchautomatic compression gain of frequency segment speech signals has beendescribed, for example, by E. Villchur, Signal Processing to ImproveSpeech Intelligibility in Perspective Deafness, J. Acoust. Soc. Am. 53,1647-1657, (1973). While this technique does provide improvement inunderstanding of speech by persons with sensori-neural deafness, it doesnot provide for the discrimination between weak speech segments and weaknoise segments providing increased gain for the speech segments but notthe noise segments, as does the present invention.

It is well known that persons wearing hearing aids withmicrophones,either non-directional or so-called directional located on or near thehead of the listener, have difficulty in understandidng speech when in aconference or other situation where several speech or other competingauditory signals reach the listener at about the same time. Thisdifficulty can be partly overcome by orienting the listenersmicrophones, especially if they are of the directional type, asdescribed, for example, in US. Pat. No. 3,770,911, so that they pick upthe desired signal to a greater extent that the undesired signalsbecause of acoustical reasons. An additional advantage, however, can beprovided if the listener were to place a microphone nearer the source ofthe desired signal which would increase the intensity of this signal atthe microphone pick-up relative to that of the other signals that arepresent. Under many social circumstances it would be appropriate toaccomplish this without obvious and awkward movements on the part of thelistener using a hearing aid with such a movable microphone. I

A common problem of hearing aids that are designed to provide largeamounts of signal gain for persons with unusually large amounts ofhearing loss is that some of the output of the earphones of the hearingaid leaks or feeds back either by air or by mechanical paths, to themicrophone of the hearing aid. This feedback causes a cyclicreamplification or oscillation that leads to complete overloading of thehearing aid causing it to squeal and be obnoxious and useless to theuser. A procedure for reducing a related type of oscillation, but in thedifferent application and requirements of publicaddress systems operatedin a reverberant room, has been described by M. R. Schroeder,Improvement of Acoustic-Feedback Stability by Frequency Shifting, J.Acoust. Soc, 36, 1718-1724, (1964).

In this procedure, the airborne signal picked up by the microphone isshifted, by well-known modulation techniques, either upwards ordownwards by about 5 to Hz before it is presented to the acoustic outputtransducers or loudspeakers of the public address system. This shift infrequency is not sufficient significantly to interfere with the audiblequality of the signal, particularly if the signal is speech, coming fromthe loudspeakers but does allow the output signal to reach levels about10 dB higher without causing feedback oscillation than is possiblewithout the application of the frequency shift processing. Thisfrequency shifting process, properly critically adapted, has notheretofore been utilized for the prevention or reduction of either themechanical linkage or the acoustic airborne feedback that may be presentin such hearing aids. Indeed, it is to be noted that in earlevel hearingaids wherein the microphone and earphone are mounted in the same case orare mechanically linked through tubes or wires, the oscillation presentin high-gain hearing aids is more often caused by the mechanicalvibrations than the airborne. It is readily appreciated, however, thatshifting the frequency coming from the earphone will tend, to asignificant degree, to prevent the vibrations in the mechanicalconnection between the earphone and microphone from progressivelyenlarging, that occurs when the gain of the amplifier of the hearing aidis cyclically reapplied to the same input/output frequency. In brief,the input signal cannot be added to itself following amplification bythe hearing aid and feedback, as normally can cause oscillation, becausethe signal is changed in frequency each time it passes through thehearing aid system and will therefore have a waveform, of feedback, thatis not consistently in phase with the input waveform as is required,within limits, to cause oscillation of the system.

An object of the present invention, accordingly, is to provide a new andimproved method of and apparatus for electronic hearing aiding thatshall not be so subject to the above-described limitations anddisadvantages of prior techniques, but that, to the contrary,significantly increases noise vs. speech signal discrimination,particularly useful for sensori-neural deafness problems and the like.

In summary, the present invention provides real-time operation withspecial automatic gain control signal processing for both the overallsignal and also for different parts of the speech spectrum in ways thatcan be adjusted to best suit the needs of individual sensorineuraldeafened ears that suffer different degrees and patterns, as a functionof frequency, of hearing deficiencies. The aid of the invention providesmeans of inserting one or more fixed increases in linear gain tosegments of the speech signal that fall below given levels relative tothe gain provided to segments that fall above given levels. The amountof increased gains and the given levels below which they are-to beinserted may be set separately for each of the different parts filteredfrom the speech spectrum. Further, the invention will automaticallydiscriminatebetween segments. of the signal that constitute speechsounds and thosesegments consisting of background noise and will applyextra gain to the speech semgents, but not to the noise segments. Theinvention also provides for so-called biear listening where thetreatment of the signal for each of the ears of the listener can besomewhat different, and further provides for pick-up, if desired, by twomicrophones of a stereo signal, in order to. utilize .the informationfound in so-called phase differences'between speech and other signals aspresent at two microphones; one placed at the position orpick-up'region'of each ear. Further, the hearing aid of theinvention-provides for an optional remote microphone that can be usedfor pick-up of signals at points at a farther-thannormal distancefrom'the user, i.e. closer to the sound source. Further, the aid of theinvention provides for an optional electronic frequency-shift of thesignal picked up at the microphone so that the signal output at theearphones is at a somewhat different frequency (about 10 Hz) than thesignal picked up by the microphone either by airborne or mechanicalagitations.

The invention will now be described with reference to the accompanyingdrawing,

FIG. 1 of which is a block diagram of a preferred apparatus embodyingthe invention; and

FIG. 2 is a similar diagram of a suitable NLGC (nonlinear-linear guncontrol)'apparatus for use in the system of FIG. 1.

In addition to the optional remote microphone, solabelled, twomicrophones (left and right) are indicated in FIG. 1 as the typicalpick-up sources of the signal input, although the system could operatewith even but one microphone. While the microphones may benondirectional, they are preferrably of the directional type,'

tional remote" microphone may be worn strapped to the users wrist sothat it appears as a wrist watch or bracelet and can be placed closer toa desired signal source by movement of his hand and'arm, or it maybeincapsulated in a pen or pencil type case, not shown,

that can'be laid on a conferencetable with a retractable cord extendingto the hearing aid amplifier. The amplifiers, batteries and associatedelectronics of the apparatus; moreover, rnay beenclosed in a case wornin a clothes pocket of the user o'r attached to his or her body orclothing in anyconvenient manner.

The major purpose of the AGC circuit is to adjust the over-all speechsignal to an intensity level for the filtering and additional automaticgain control processes to follow, such that the subsequent system willnot overload, but yet will be at a level adequate to give proper signaltransmission. Generally, at a distance of about three feet from atalker, the weaker speech sounds in conversational speech are at a levelof about 20 dB re 0.0002 microbar, and the more intense speech sounds ina conversational speech signal are of the order of 60 dB. A dynamicrange of about 40 dB is accordingly present in a speech signal utteredat a constant and conversational level of effort. When the listener iscloser to the talker, furthermore, or when the talker uses a higher thannormal effort of speaking, the level of the speech signals may go up to100 decibels or so.

The AGC circuit is adjusted to provide a decrease in gain when thesignal envelope is above a specified level (typically 60 dB) forapproximately 0.001 seconds. Conversely, whenever the envelope levelfalls below a specified level (typically 60 dB) for approximately 0.02seconds, the gain of the system automatically assumes its normal stateof gain and treats signals between about 20 to 60 dB input in asubstantially linear fashion. The decreases in gain effected by AGC areproportional to the degree to which the speech envelope (averaged overabout 0.001 second) exceeds the level equivalent at that point in thesystem to a speech level of about 50 dB at the input to themicrophone(s). Thus, AGC adjusts the average gain so that speech at anintensity greater than about 50 dB at the microphone(s) will generallybe placed within the optimum operation region of the filters andassociated electronic components that follow, as hereinafter described.

The signal from AGC may be fed to the frequency shifter section, ifused, shown at PS, prior to being fed to Sections 1, 2, 3, and 4 of thehearing aid as shown in FIG. 1. Such a frequency-shifter FS, by means ofstandard RF modulation and demodulation techniques, as of the typedisclosed in said Schroeder article, for example, shifts the frequencyspectrum of the signal picked up at the microphone by about Hz.Accordingly, the frequency spectrum coming from the earphones of thehearing aid is shifted from its location on the frequency spectrum, fromthe location it occupied when picked up by the microphone, increasing byabout 10 dB the tolerable level of the level of output from theearphones that can be reached before acoustic feedback between theearphones and microphones causes oscillation in the hearing aidamplifiers. It is to be further noted that this frequency shiftingprocess will also tend significantly to reduce the vibrations that maybe set up in the mechanical linkage between the earphone(s) andmicrophone(s) of the hearing aid, said vibrations, if sufficientlystrong, being a sourceof causing oscillation and overload in th hearingaid.

The signal from AGC or the optional frequency shifter is fed to andprocessed by Sections 1, 2, 3, and 4 of the hearing aid, as shown inFIG. 1. Section 1 passes a broad band of frequencies and each ofsections 2, 3 and 4 contains a narrow band filter of different adjacentbands, as later explained. Section 1 transmits the broadband signal overthe range of about 200 Hz to 7000 Hz to the listener, with adjustment ofits level made possible by means of variable gain amplifiers 1A, 1B, LE,and RE. To this broad-band signal are added the outputs of Sections 2,3, and 4, which have broadly similar functions but are individuallyadjustable in several regards. The purpose of these sections is toseparate or filter the speech or other acoustic signals into relativelynarrow bands of frequencies so that the respective bands can beamplitude-processed and gainadjusted in ways that will enhance theunderstandability of speech and other acoustic signals for persons withsensori-neural deafness. As indicated above, in certain regions, usuallythe higher frequency regions, the ear with sensori-neural deafness willusually have a usable dynamic range of but 10 dB or so between levelsthat are inaudible and levels that overload the ear, as compared with ausable dynamic range of more than 60 dB for the normal ear. At otherfrequency regions, the dynamic range may be greater or less, dependingon the particular pattern of damage to the sensori-neural receptors inthe inner ear. The purpose of the Sections 2, 3, and 4 is to provide themeans of processing the different frequency bands of speech to thedegree and in the way best suited for the hearing characteristics of agiven ear suffering sensori-neural deafness, and to add these speciallyprocessed frequency bands to the normal, broadband signal beingtransmitted by Section 1, as shown in FIG. 1. It is to be understoodthat for some sensori-neural deaf ears, fewer or more than four suchsections of signal processors will be required, or that the bandwidthsof one or more of the sections indicated may be changed, and that thenon-linear gain control processing to be later described may beinactivated in given sections.

The description to follow of the functioning of Section 4 of FIG. 1, forexample, will suffice to explain also the operation of Sections 2 and 3,except that the frequency-bands, the amplitude levels to which the gainis specially adjusted, and the following gain settings may be atdifferent values for each section.

The band-pass filter of Section 4 separates the energy in the frequencyband 2500 to 7000 Hz from the total spectrum of the signal. The outputof this band-pass filter is then passed through a nonlinear-linearcompresser gain control (NLGC). The amount of signal compression is setto be suited to the loss in a given ear in dynamic range of hearingability for sounds in the frequency band 2500 to 7000 Hz. The NLGCoperates such that when the signal is, for about 0.005 seconds, below agiven level, an additional amount of signal energy is applied to thesignal energy in the frequency band 2500 to 7000 Hz.

The output of this NLGC circuit is then further amplified in separatesplit paths containing amplifiers 4A and 48, if necessary, to meetpossible differences in sensitivity between the left and right ears ofthe listener.

Sections 2 and 3 are also individually separately adjusted to providethe degree, if any, of signal dynamic range compression best suited foroptimizing the reception and understandability of signals, especiallyspeech, as determined by the contributions of the several respectivedifferent frequency bands 750-1500 Hz and 1500-2500 Hz. The outputs ofthese three sections are then split into pairs and combined throughresistor networks with the broad-band signal from Section 1 forpresentation to the listener, with all the right and left ear paths ofthe NLGC outputs being connected together, respectively.

It is to be understood that the specific number of filter sections andthe cut-off frequencies given in FIG. 1

are illustrative only, and that greater or fewer sections and differentcut-off frequencies may be used in various specific applications of thisinvention. Further, the use of separate output amplifiers for each ofthe two ears is often not required, because both ears of a personsuffering sensori-neural deafness often have similar characteristics.

In accordance with the present invention, the NLGC part of the hearingaid, with its speech-noise discrimination operation, may be of the formillustrated in FIG. 2 for operation, for example, in Section 4 ofFIG. 1. In FIG. 2, when the input signal envelope is between 50 to 60 dBor greater, Gate 1 remains closed and these time segments of the signalpass directly through towards the output, so-labelled, without theemphasis or extra gain available from amplifier B. When the signalenvelope falls to a value indicating that the input signal is below 50dB, Gate 1 opens and the signal in Path B (which has been amplified byamplifier B by a given amount relative to the level in Path A) is addedto the signal present in Path A, provided that Gate 2 is also open. Gate2, by means of the attack and release time control 2' is open when thesignal envelope is more than the illustrative 50 dB; however, when thesignal remains below 50 dB for more than 0.5 sec., Gate 2 closes,thereby preventing further gain-emphasized signal segments comingthrough Gate 1 from reaching Path A. Accordingly, the extra emphasis oramplification given to the signal by amplifier B is not added to thesignal in Path A. Amplifier B is adjustable so that the amount of extraemphasis given to the signal, relative to its level in Path A, can bevaried to best meet the needs of different degrees of hearing loss.

Rectifier R, amplifier 1" and attack-release time control elements 1 and2' perform the following functions: rectifier R provides a means ofmaking the negative parts of the signal continuum positive in voltage;and amplifier l is adjustable and provides a means for adjusting therectified signal continuum level reaching the attack-release timecontrols 1' and 2. Accordingly, depending on the desires of the userduring a given input signal-noise condition, the signal continuum levelcan be increased or decreased from rectifier, R, so that theattack-release controls 1 and 2', which affect Gates 1 and 2,respectively, and which are set to operate at specified voltages, willbe activated with different signal-continuum levels at the input to themicrophones. Thus, amplifier 1 "provides a means of causing the gates tobe activated with lesser or greater input signals at the microphone aswill be advantageous to persons with different degrees of sensori-neuraldeafness.

The purpose of the described double-gate action is to give the weaker,short duration (less than 0.5 sec) segments of the signal the extraamplification or emphasis relative to the strong intensity segments ofthe speech signal; but not to give this extra amplification torelatively low intensity background noise which is typically moresteady-state than the speech signal. This background noise may continueat a level below, say, 50 dB for much longer duration than 0.5 sec. andis especially objectionable to persons using hearing aids that pro-'vide automatic gain compression that typically increases the relativeintensity of this background noise.

It is to be understood that for some types of speech or other signals,the attack and release times for the operation of Gates 1 and 2 may bechanged for optimum results from those specified in FIG. 2. It is alsoto be noted that the NLGC processing system herein described has otherpossible applications beyond that in hearing aids where it is desirableto provide relative emphasis or de-emphasis to different segments ofelectronic signals that dynamically vary in intensity in somewhatpredictable ways such that its use, while particularly adapted to thepresent invention, is also applicable in other signal processing systemswherein similar advantages are sought.

Suitable components for the circuit elements are as follows: Gates 1 and2 may, for example, be of the Field Effect Transistor (FET) type,described in Electronic Principles by Malvino, McGraw-I-Iill, New York,1973: attack and release time control circuits 1 and 2 may be of theoperational amplifier type with appropriate capacitive and resistivefeedback elements, as described in the same publication. Clearly, othertypes of well-known circuits may be similarly employed, and furthermodifications, within the spirit and scope of the invention, willsuggest themselves to those skilled in this art.

What is claimed is:

l. A method of aiding hearing, that comprises, adjusting the over-allintensity level of speech signals with substantially linear gain over apredetermined range of intensities; applying the adjusted-intensitysignals along a plurality of frequency filtering paths, one passing abroad band of the speech signal frequencies, and the others passingsuccessive adjacent narrow bands within said broad band; reducingseparately in each of the other paths, the dynamic range of intensitylevels corresponding to segments of speech signals that vary inintensity for brief moments in the corresponding narrow bands, asdistinguished from the more steady state segments of background noiseand steady-state signals; and combining the signals from said paths.

2. A method as claimed in claim 1 and in which the signals in each ofsaid paths are split and fed along a pair of further paths for right andleft ear excitation, with the signal combining step being effectd bycombining the right ear further paths and separately combining the leftear further paths.

3. A method as claimed in claim 2 and in which independent leveladjustments are effected in each of the further paths prior to suchcombining.

4. A method as claimed in claim 1 and in which the speech signals arederived from a pair of right and left ear acoustic pick-up regions and afurther pick-up region adjustable closer to the source of speech, andthen the same are combined prior to said over-all intensity leveladjusting step.

5. Hearing aid apparatus having, in combination, microphone pick-upmeans; automatic gain control means connected with the pick-up means toadjust the overall signal intensity level of speech signals withsubstantially linear gain over a predetermined range of intensities; aplurality of filter paths connected with the automatic gain controlmeans and comprising a first path with broad band filter means for thespeech signal frequencies and a plurality of further paths containingband-pass filters of successive adjacent narrow bands within said broadband; a plurality of speech-noise discrimination means, one connected ineach of the plurality of further paths for separately reducing in eachpath the dynamic range of signal intensity levels corresponding tosegments of speech signals that vary in intensity for brief moments inthe respective narrow bands, as distinguished from the more steady statesegments of background noise and steady-state signals; and means forcombining the signals from said paths.

6. Apparatus as claimed in claim and in which said combining meanscomprises pairs of right and left ear paths, each pair split from theoutput of the broad band filter means and the outputs of the pluralityof speechnoise discrimination means, with all right ear paths connectedtogether and all left ear paths connected together.

7. Apparatus as claimed in claim 6 and in which said pairs of pathscomprise separate variable gain amplifier means and resistive combiningnetworks.

8. Apparatus as claimed in claim 7 and in which further variable gainamplifier means is provided at the output of each of theconnected-together right and left ear paths, independently operable withrespect to the said separate variable gain amplifier means.

9. Apparatus as claimed in claim 4 and in which each of saidspeech-noise discrimination means comprises a pair of signal processingpaths connected to the corresponding band pass filter means, one of saidpaths including gating means and attack-release time control means foroperating the gating means to apply amplification emphasis along theother processing path for the weaker short-duration segments of thesignal relative to the strong intensity segments of the speech signal,but without providing added amplification to relatively low intensitybackground noise.

[0. Apparatus as claimed in claim 4 and in which said microphone pick-upmeans comprises right and left ear microphones and a remote microphoneadjustable to regions closer to the source of speech, with all of themicrophones connected to the automatic gain control means.

11. Hearing aid apparatus having, in combination, right and left earmicrophone pick-up means, remote microphone means adjustable to regionscloser to the source of sound, and common automatic gain control meansconnected to all of said microphone means to receive the combined inputsthereof.

12. Hearing aid apparatus as claimed in claim 5 and in which frequencyshift means is provided connected with the automatic gain control meansfor shifting the frequency of signals picked up by the microphone pickupmeans and mechanical vibratory linkages associated therewith, saidfrequency shift means comprising means for modulating with one frequencyand demodulating with a second and different frequency.

1. A method of aiding hearing, that comprises, adjusting the over-allintensity level of speech signals with substantially linear gain over apredetermined range of intensities; applying the adjusted-intensitysignals along a plurality of frequency filtering paths, one passing abroad band of the speech signal frequencies, and the others passingsuccessive adjacent narrow bands within said broad band; reducingseparately in each of the other paths, the dynamic range of intensitylevels corresponding to segments of speech signals that vary inintensity for brief moments in the corresponding narrow bandS, asdistinguished from the more steady state segments of background noiseand steadystate signals; and combining the signals from said paths.
 2. Amethod as claimed in claim 1 and in which the signals in each of saidpaths are split and fed along a pair of further paths for right and leftear excitation, with the signal combining step being effectd bycombining the right ear further paths and separately combining the leftear further paths.
 3. A method as claimed in claim 2 and in whichindependent level adjustments are effected in each of the further pathsprior to such combining.
 4. A method as claimed in claim 1 and in whichthe speech signals are derived from a pair of right and left earacoustic pick-up regions and a further pick-up region adjustable closerto the source of speech, and then the same are combined prior to saidover-all intensity level adjusting step.
 5. Hearing aid apparatushaving, in combination, microphone pick-up means; automatic gain controlmeans connected with the pick-up means to adjust the overall signalintensity level of speech signals with substantially linear gain over apredetermined range of intensities; a plurality of filter pathsconnected with the automatic gain control means and comprising a firstpath with broad band filter means for the speech signal frequencies anda plurality of further paths containing band-pass filters of successiveadjacent narrow bands within said broad band; a plurality ofspeech-noise discrimination means, one connected in each of theplurality of further paths for separately reducing in each path thedynamic range of signal intensity levels corresponding to segments ofspeech signals that vary in intensity for brief moments in therespective narrow bands, as distinguished from the more steady statesegments of background noise and steady-state signals; and means forcombining the signals from said paths.
 6. Apparatus as claimed in claim5 and in which said combining means comprises pairs of right and leftear paths, each pair split from the output of the broad band filtermeans and the outputs of the plurality of speech-noise discriminationmeans, with all right ear paths connected together and all left earpaths connected together.
 7. Apparatus as claimed in claim 6 and inwhich said pairs of paths comprise separate variable gain amplifiermeans and resistive combining networks.
 8. Apparatus as claimed in claim7 and in which further variable gain amplifier means is provided at theoutput of each of the connected-together right and left ear paths,independently operable with respect to the said separate variable gainamplifier means.
 9. Apparatus as claimed in claim 4 and in which each ofsaid speech-noise discrimination means comprises a pair of signalprocessing paths connected to the corresponding band pass filter means,one of said paths including gating means and attack-release time controlmeans for operating the gating means to apply amplification emphasisalong the other processing path for the weaker short-duration segmentsof the signal relative to the strong intensity segments of the speechsignal, but without providing added amplification to relatively lowintensity background noise.
 10. Apparatus as claimed in claim 4 and inwhich said microphone pick-up means comprises right and left earmicrophones and a remote microphone adjustable to regions closer to thesource of speech, with all of the microphones connected to the automaticgain control means.
 11. Hearing aid apparatus having, in combination,right and left ear microphone pick-up means, remote microphone meansadjustable to regions closer to the source of sound, and commonautomatic gain control means connected to all of said microphone meansto receive the combined inputs thereof.
 12. Hearing aid apparatus asclaimed in claim 5 and in which frequency shift means is providedconnected with the automatic gain control means for shifting thefrequency of signals picked up by the microphone pick-up means aNdmechanical vibratory linkages associated therewith, said frequency shiftmeans comprising means for modulating with one frequency andde-modulating with a second and different frequency.